Biogenesis of the platelet receptor for fibrinogen: evidence for separate precursors for glycoproteins IIb and IIIa.

Congenital absence of platelet glycoproteins IIb and IIIa (GPIIb and GPIIIa) results in a severe bleeding disorder characterized by defective platelet aggregation and failure of fibrinogen to bind to platelets. GPIIb is a two-chain protein containing disulfide-linked alpha and beta subunits. GPIIb and GPIIIa are present as a heterodimeric, noncovalent complex in the platelet plasma membrane and function as the fibrinogen receptor. To characterize synthesis of these two proteins, RNA isolated from a human leukemia cell line that contains GPIIb and GPIIIa was translated in a wheat germ cell-free system. Polyclonal antibodies specific for each protein immunoprecipitated distinct [35S]methionine-labeled precursors, indicating that GPIIb and GPIIIa are translated from separate mRNAs. Moreover, using specific antibodies against either intact unreduced GPIIb or the beta subunit, we obtained evidence for synthesis of a common polypeptide precursor for GPIIb alpha and GPIIb beta. Based on experiments using microsomal membranes, it appears that GPIIb is integrated into the platelet membrane with little or no cytoplasmic component. These results suggest that precursors of GPIIb and GPIIIa may be encoded by separate genes and that each precursor is processed before delivery to the plasma membrane.     

Membrane assembly in vitro: Synthesis, glycosylatio, and asymmetric insertion of a transmembrane protein

Membrane assembly was observed to proceed in cell-free extracts. Specifically, the membrane glycoprotein of vesicular stomatitis virus was synthesized in crude extracts of wheat germ in the presence of membrane vesicles derived from pancreatic endoplasmic reticulum. The resulting glycoprotein spans the lipid bilayer asymmetrically, is glycosylated, and is indistinguishable in these respects from the form of the glycoprotein found in the rough endoplasmic reticulum of virus-infected cells. Both glycosylation and asymmetric transmembrane insertion of the glycoprotein into membranes in vitro require protein synthesis in the presence of membranes. The carboxyl-terminal 5% of the polypeptide chain is located on the external surface of vesicles, corresponding to the cytoplasmic surface of the endoplasmic reticulum in cells. Most, or all, of the amino-terminal portion of the glycoprotein, as well as the protein-bound carbohydrate, appears to be located within the lumen of the membrane vesicles. These findings demonstrate that insertion of this membrane protein occurs during or immediately after protein synthesis. The results are consistent with the concepts that the growing membrane protein is extruded across the endoplasmic reticulum membrane amino terminus first and that glycosylation is restricted to the lumenal surface of the membrane. The cell-free system reported here should prove valuable for studying these processes.     

Levodopa Dose Equivalency in Parkinson's Disease: Updated Systematic Review and Proposals

Background

To compare drug regimens across clinical trials in Parkinson's disease (PD) conversion formulae between antiparkinsonian drugs have been developed. These are reported in relation to levodopa as the benchmark drug in PD pharmacotherapy as ‘levodopa equivalent dose’ (LED). Currently, the LED conversion formulae proposed in 2010 by Tomlinson et al. based on a systematic review are predominantly used. However, new drugs with established and novel mechanisms of action and novel formulations of longstanding drugs have been developed since 2010. Therefore, consensus proposals for updated LED conversion formulae are needed.

Objectives

To update LED conversion formulae based on a systematic review.

Methods

The MEDLINE, CENTRAL, and Embase databases were searched from January 2010 to July 2021. Additionally, in a standardized process according to the GRADE grid method, consensus proposals were issued for drugs with scarce data on levodopa dose equivalency.

Results

The systematic database search yielded 3076 articles of which 682 were eligible for inclusion in the systematic review. Based on these data and the standardized consensus process, we present proposals for LED conversion formulae for a wide range of drugs that are currently available for the pharmacotherapy of PD or are expected to be introduced soon.

Conclusions

The LED conversion formulae issued in this Position Paper will serve as a research tool to compare the equivalence of antiparkinsonian medication across PD study cohorts and facilitate research on the clinical efficacy of pharmacological and surgical treatments as well as other non-pharmacological interventions in PD. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.